Well, the 50mm f/1.8 II was pretty much *the* must-have lens, if only because it's a "my first fast-prime" and for $100, why not?

if the 40/2.8 Pancake launches at $200, it won't be more than a year before it's $150 or even lower, it could take over that position of "my first cheap-as-anything prime".
I've never seen a xxxxD/xxxD/rebel kit with a 50/1.8II (except maybe on ebay), but something like an 1100D with a 40/2.8 would make a very nice walkaround/street/discrete kit, methinks (I'm waiting for the first shots from/of a 40/2.8 pancake on a 1DX though)